This invention relates to industrial baghouses and, more particularly, to an improved cage, bag and diffuser assembly for a baghouse to facilitate maintenance.
Continuous emphasis on environmental quality has resulted in increasingly strenous regulatory controls on industrial emissions. One technique which has proven highly efficient in controlling air pollution has been the separation of undesirable particulate matter from a gas stream by fabric filtration.
Such filtration is carried out in dust collection apparatus known in the trade as a "baghouse" which operates on the same general principle as an ordinary household vacuum cleaner, except on a much larger scale. Basically, the baghouse is a sheet metal housing divided into two chambers, referred to as plenums, by one or more tube sheets. Disposed within openings communicating with the plenums are fabric filters. A particle-laden gas stream, induced by the action of a fan, blows into one chamber (dirty air plenum) wherein dust accumulates on the fabric filter as the gas passes through the fabric into the other chamber (clean air plenum) and out the exhaust.
Although all baghouses are designed in accordance with the foregoing general principles, there are numerous operational and structural distinctions. The present invention relates to a baghouse wherein the dirty and clean air plenums are separated by a tube sheet having a plurality of vertically suspended filter bags in which cylindrical wire cages are inserted for skeletal support. Filtration of the process gas occurs from outside to inside of the bags. As a result, baghouses constructed in this manner are normally referred to as outside bag collectors.
During continuous operation of the baghouse the bags must be periodically cleaned to remove the filter cake which is deposited on the fabric. One of two cleaning methods are typically employed in an outside bag collector. In pulsing-plenum cleaning, high pressure air is introduced to a compartment in the clean air plenum and the dust cake is simultaneously dislodged from all filter bags in that compartment. With pulse jet cleaning, on the other hand, the upper end of the cage is equipped with a diffuser, venturi or nozzle directed interiorly of the cage and a short blast of high pressure air or a discharge of low pressure, high volume air is individually introduced through each nozzle. This pulse of air travels through the filter sleeve to dislodge the dust cake.
Further details of a baghouse having cages and bags suspended from a tube sheet and employing pulse jet cleaning may be found in U.S. Pat. No. 3,876,402 by Bundy et al. issued Apr. 8, 1975, and specifically incorporated herein by reference.
It is of course inevitable that, in service, bags within the baghouse will develop leaks or ruptures which impair the efficiency in removing particulate matter from contaminated gas. However, in order to stay within industrial emissions standards and operate at peak efficiency, it is imperative that defective bags be identified and replaced. Proper baghouse maintenance is therefore of crucial importance.
The replacement of defective bags has traditionally been a time-consuming and costly maintenance operation requiring at least two workmen. With the top access door of the clean air plenum removed, the bag to be replaced is first located. Next the venturi or nozzle, such as venturi 40 in FIG. 5 of U.S. Pat. No. 3,876,402, is removed and laid aside for installation later. The clamping band, such as band 60 of U.S. Pat. No. 3,876,402, is then removed from around the tube sheet collar, such as collar 31(b) of the foregoing patent. The cage and bag, such as cage 41 and bag 42 of U.S. Pat. No. 3,876,402, is then lifted from the tube sheet and one workman lowers the bag and cage assembly to a second workman on the ground. The filter sleeve is then stripped off of the cage. At this phase of the maintenance operation it is common to encounter bags which are fused to the cage and require additional stripping time. Once the bag is removed, however, the cage is thoroughly inspected for corrosion and sharp edges that may tear or weaken the filter fabric. Such areas must be filed or brushed to present a smooth surface once again. Cages in bad condition must be replaced altogether. A new filter sleeve is then carefully fitted over the reconditioned or replaced cage and the assembly is again raised to the top of the baghouse. The assembly is carefully lowered into the appropriate opening in the tube sheet and the fabric filter is folded over the tube sheet collar and encircled with a clamping band. Finally, the venturi or nozzle is replaced and should be carefully aligned in the opening to the cage-bag assembly. Any misalignment of the nozzle will naturally misdirect the cleaning pulse during subsequent cleanings and can result in premature bag failure and inefficient cleaning.
As apparent from the foregoing description, the prior art maintenance procedure is not only costly and time-consuming but, when properly done, requires exceptional care. Accordingly, there is a long felt need in this industry for improvements in bag, cage and nozzle assemblies to alleviate the many maintenance difficulties which have been encountered in the past. The primary goal of this invention is to meet this need.
More specifically, an object of this invention is to provide an improved cage, bag and nozzle assembly to decrease maintenance time and costs heretofore associated with baghouse operations. Also, the improved assembly achieves the added advantage of prolonging bag life which; in turn, results in a less rigorous maintenance schedule. These advantages are achieved by provision of a unitary and preassembled filter cartridge which replaces the three separate components comprising the bag, cage and diffuser in a conventional outside bag collector and which may be discarded entirely when it is necessary to replace the filter.
Another object of the invention is to provide an integrally joined cage and nozzle construction which rigidly maintains alignment of the nozzle with respect to the cage to insure proper jet pulse cleaning after installation. As an adjunct to this object, the improved construction eliminates the requirement of a separate collar to be spot welded on the upper cage assembly for tensioning the bag as has been practiced by this industry.
Another object of the invention is to provide an improved cage fabricated of wire mesh chosen to optimally support a filter bag yet still permit unrestricted flow through the filter. High speed photography tests reveal that the wire cage can actually block air flow through the filter and thus impair efficiency of the baghouse in both filter and cleaning operations. To minimize this effect, it has been determined that a wire mesh cage is best fabricated from wire 1/2 inch to 1 inch by 1 inch to 2 inches in the range of 14 to 16 gauge wire.
An additional object of this invention is to provide a filter cartridge of the character described which is self-sealing upon installation in the tube sheet. Thus, the prior time-consuming methods of carefully folding the bag itself over the tube sheet collar or the even more undesirable situation of requiring a sealing material separate of the bag are outmoded by this filter cartridge. The preassembled cartridge is simply inserted in the tube sheet and press fitted to the collar of the tube sheet to achieve an effective seal thereagainst.
A further object of the invention is to provide an improved bag construction for an outside bag collector to permit utilization of a filter sleeve of cylindrical construction having two open ends. Outside bag collectors have traditionally employed a cylindrical bag having a sewn in bottom. Naturally this feature requires a separate manufacturing operation during the bag fabrication. Inasmuch as all of the process gas in a baghouse must necessarily flow past the bottoms of the bags, the bottom is a point of high wear and acts as a baffle within the dirty air plenum. In one embodiment of this invention, the bag is constructed from a cylindrical sleeve having a specially designed reinforced cuff which seals against the bottom pan or end closure secured to the bottom of the cage.
Yet another object of the invention is to provide an improved end cap for a cage assembly to be used with filter sleeves having an open bottom. When an open bottom filter sleeve has been used in this industry, it is necessary that the bottom of the cage be fitted with an end cap. The end cap may be welded to the bottom of the cage and the filter sleeve is secured thereto by means of an adjustable clamping band such as a conventional hose clamp. The construction of this invention provides for an end cap which may be crimped onto the cage without splitting or otherwise rupturing the end cap as has been encountered in crimping operations.
Other and further object of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description of the drawings.